Aircraft fuel tank inerting and venting systems are used as means to prevent fuel tank explosions by reducing the oxygen concentration in the ullage. This reduces the likelihood of an explosion even if fuel vapors and ignition source are present, as the low oxygen concentration cannot sustain flame propagation or an explosion. A typical aircraft inerting and venting system may consist of a means to generate nitrogen enriched air (NEA), a means to control the oxygen concentration of NEA, and a means to distribute the NEA in the fuel tank ullage during all phases of the flight.
For some inerting and venting systems, there remain various shortcomings, drawbacks, and disadvantages relative to certain applications. Some inerting and venting systems inject NEA directly into fuel tank compartments through NEA injectors with the aim to a) deliver a certain amount of NEA mass flow at a given oxygen concentration into the fuel tank depending on aircraft operating conditions, and b) achieve gas mixing within the ullage space. U.S. Pat. Nos. 7,204,868, 7,442,230; and 8,777,165 disclose injecting NEA either directly into the fuel tank vent lines to inert ambient air prior to flowing into the fuel tank during aircraft descend, instead of reducing the oxygen concentration of the inflowing vent air after it has entered the fuel tank via fuel tank internal NEA jets. The former approach prevents excessively high oxygen concentrations and assures a more uniform gas mixture in the ullage space, reducing the likelihood oxygen concentration “hot” spots, at the same amount of admitted NEA. In U.S. Pat. No. 8,074,932, NEA from the inerting and venting system is also admitted to the vent air flow upstream of the fuel tank, however, in this case it is not injected directly into the vent lines but admitted to a mixing chamber added to the aircraft vent system between the aircraft surge tank and the wing tanks. NEA flow is regulated via valves which can be controlled by oxygen sensors in the ullage space.
Thus, while the afore mentioned inerting and venting systems describe ways to manage oxygen concentration within an aircraft fuel tank through controlled admission of NEA into the ullage space or into the tank vent system or a mixing chamber as part of the vent system, these systems do not address the effect of the NEA flow on tank pressure and vice versa directly. The inerting and venting systems do not provide simultaneous or direct passive control of both mass flow and oxygen concentration of mass flow entering the full tank ullage space, nor across different fuel tank compartments where differing ullage space conditions may be present. Existing systems and methods merely regulate and control tank pressure by the vent line system independently for example via a climb-and-dive valve (closed vent line system) and the residing pressure conditions, while the NEA flow is actively controlled for example by oxygen sensors in the fuel tank ullage space. There is no provision for passively adjusting usage of available NEA flow across different fuel tank compartments based on for example changing oxygen concentration in the respective ullage spaces or in the NEA supply.
Accordingly, there remains a need for further contributions in this area of technology.